Chemical mechanical polishing (CMP) is a well-accepted technique to planarize the surface of a semiconductor wafer during fabrication of a semiconductor device. In CMP, planarization is achieved by chemically and mechanically removing uneven topographic features of a semiconductor wafer. A conventional CMP technique involves polishing the surface of a semiconductor wafer with a rotating polishing pad using slurry of colloidal particles in an aqueous solution, which is supplied to the surface of the polishing pad through a slurry delivery line. The slurry promotes planarization of the wafer surface by producing a chemical reaction with the wafer surface and by providing abrasives to “grind” the wafer surface with the polishing pad.
Some conventional CMP apparatuses include a single small polishing pad to polish one semiconductor wafer at a time. Other conventional CMP apparatuses include a number of small polishing pads to polish multiple semiconductor wafers in parallel to increase the throughput of the apparatus. However, the CMP apparatuses with multiple polishing pads require a large footprint, due to the large area required by the multiple polishing pads. Thus, CMP apparatuses have been developed that include a large polishing pad to simultaneously polish multiple semiconductor wafers. These multi-wafer CMP apparatuses typically utilize a wafer transfer carousel with a number of wafer carriers, so that multiple semiconductor wafers can be polished on the large polishing pad using appropriate slurry.
During fabrication of a semiconductor device, a semiconductor wafer may have to be repeatedly CMP planarized. Depending on the features of the semiconductor wafer being polished, the sequential CMP planarizations may need to be performed using different types of slurries. For example, when a semiconductor wafer is polished using slurry with coarse abrasives, the polished wafer may be further polished using slurry with finer abrasives to achieve a smoother planarized wafer surface. As another example, for fabrication of Cu interconnections, a first type of slurry is used to polish Cu plugs, and then, a second type of slurry is used to polish Ta or TaN barrier layer. When different types of slurries are used for sequential CMP planarizations, each planarization is typically performed on a different CMP apparatus. Thus, multiple CMP apparatuses may be needed to sequentially planarize each semiconductor wafer.
A concern with the use of multiple CMP apparatuses for the sequential CMP planarizations is that the CMP apparatuses may require a substantial amount of valuable real estate in a cleanroom. Another concern is that semiconductor wafers must be transferred from one CMP apparatus to the next CMP apparatus, which can delay the overall fabrication process.
In view of the above concerns, there is a need for a CMP apparatus and method for sequentially polishing semiconductor wafers on a single polishing pad using different types of polishing solutions so that additional CMP apparatuses are not needed, and consequently, the amount of space required in a cleanroom can be significantly reduced.